This invention is directed to preventing the likelihood of permanent breakdown of reverse junctions in MOS semi-conductor integrated circuits, and in particular, to preventing secondary breakdown of the reverse junction of a C-MOS integrated circuit in response to unexpected high voltages being applied thereto.
The problem of breakdown voltages applied to gate electrode of MOS transistors is well-known in the art. Sudden and unexpected applications of external electric fields having a high voltage and low energy density, such as static electricity to the gate electrode are likely to cause a dielectric breakdown of the reverse bias junction. Usually, the breakdown is not irreversible and upon the suddenly applied external electric field being removed, the junction returns to the normal state. Such breakdown is called "primary" breakdown and refers to a breakdown condition which is not irreversible. Nevertheless, if the external field is of a sufficiently high voltage as to effect a dielectric breakdown of the gate insulating film of the MOS transistor, a short circuit is formed between the gate electrode and the adjacent portion of the substrate previously insulated from the gate electrode by the gate insulating film (hereinafter referred to as "secondary" breakdown). Accordingly, common practice is to provide a diode clamping circuit at the gate terminal of a C-MOS circuit for absorbing any electric charge that has a voltage higher than the primary breakdown voltage to thereby prevent secondary breakdown of the gate terminal from occurring.
Although such diode clamping circuits have been effective in reducing the occurrences of secondary breakdown at the gate terminal, the reverse junctions between the drain region and output terminals in MOS devices are likely to reach secondary breakdown in response to sudden electric charges such as electrostatic fields being applied thereto. This accounts for breakdowns of the drain portion being observed in integrated circuit devices during assembly of the integrated circuitry and during transit of same.